The human immunodeficiency virus (HIV), a member of the lentivirus genus of retroviruses, is the causative agent of acquired immunodeficiency syndrome (AIDS) (review in Gonda, M. A., Ann. N. Y. Acad. Sci., 724:22-42 (1994); Gonda, et al., Control of Virus Diseases (Kurstak, E. Ed.), pp. 3-31 (1992); Gallo, R. C., J. Infect Dis., 164:235-243 (1991); Levy, J. A., Microbiol. Rev., 57:183-289 (1993)). HIV primarily infects CD4+ lymphocytes and macrophages. The depletion of CD4+ lymphocytes produces immune dysfunction that cripples the immune system's ability to fight opportunistic infections and cancers. Although extensive virologic, immunologic, and molecular characterizations have provided considerable insight into the biology and epidemiology of HIV, there are presently no effective treatments to reverse the disease process or vaccines to prevent infection. It is now known that HIV infection has reached pandemic proportions (Mann, J. M., J. Acquired Immune Def. Syndromes, 3:438-442 (1990); Piot, et al., J. Acquired Immune Def. Syndromes, 3:403-412 (1990)). Thus, the development of safe and effective vaccines and therapeutics remains a high priority in AIDS research (Gallo, R. C., Nature Medicine, 1:753-759 (1995)).
HIV is a protein-encapsidated positive-sense RNA virus that buds from the infected cell membrane. Its genome contains the obligate gag, pol and env structural genes flanked by the long terminal repeats, as well as a number of nonstructural regulatory genes (Gonda, et al., Control of Virus Diseases (Kurstak, E. Ed.), pp. 3-31 (1992); Levy, J. A., Microbiol. Rev., 57:183-289 (1993)). The gag gene encodes the Gag precursor, Pr55. The pol gene encodes proteins with enzymatic function (protease, reverse transcriptase, and endonuclease/integrase), while the env gene encodes the envelope glycoprotein precursor (gp160).
The HIV virion can be divided into two basic morphologic components: the viral core and envelope. The viral core consists predominantly of gag- and pol- encoded proteins and the viral RNA. In immature virions, the core consists primarily of uncleaved Pr55. Upon maturation of the virus, the viral protease cleaves Pr55 and products of pol into functional domains important in virus entry and replication. Pr55 is processed into the matrix (p17.sup.Gag), capsid (p24.sup.Gag), nucleocapsid (p7.sup.Gag) and p6.sup.Gag proteins. The viral envelope consists of a lipid bilayer derived from the cell surface membrane into which gp160 is specifically concentrated. gp160 is cleaved by cellular proteases into the surface glycoprotein, gp120, which interacts with the cellular receptor, and the transmembrane glycoprotein, gp41, which anchors gp120 to the plasma membrane (Gonda, et al., Control of Virus Diseases (Kurstak, E., Ed.), pp. 3-31 (1992)).
In the initial phase of infection, HIV replicates rapidly and large quantities of virus are shed from infected cells; this is accompanied by destruction of effector cells (CD4+ lymphocytes) important in developing a competent immune response (Daar, N., Engl. J. Med., 324:961-964 (1991); Graziosi, et al., Proc. Natl. Acad. Sci. USA, 90:6405-6409 (1993); Borrow, et al., J. Virol., 68:6103-6110 (1994); Pantaleo, et al., Nature (London), 370:463-467 (1994)). The initial viremia passes into a subacute phase in which the activated immune system has apparently exerted some control over virus spread (Pantaleo, supra; Koup, et al., J. Virol., 68:4650-4655 (1994)). In the majority of HIV cases, the subacute phase of infection progresses to severe disease, which includes a depletion of CD4+ lymphocytes and the subsequent onset of opportunistic infections and AIDS. A small proportion of HIV-infected individuals appears to have a reduced virus load suggesting effective immunological control of the virus (Cao, et al., N. Engl. J. Med., 332:201-208 (1995)). It is thought that neutralizing antibodies and, perhaps more importantly, cytotoxic T-lymphocytes (CTLs) specific for HIV Gag and Env antigens may be of key importance in inhibiting virus spread and delaying pathogenesis (Klein, et al., J. Exp. Med., 181:1365-1372 (1995); McFarland, etal., J. Infect. Dis., 167:719-723 (1993)). A better understanding of the immunologic mechanisms by which HIV nonprogressors survive may provide clues to the nature of an appropriate protective immune response.
It was previously thought that both viral envelope and core components were necessary to make viral particles. Recent molecular studies on virus gene expression by heterologous promoters have shown that the gag-encoded precursor of retroviruses contains the minimal particle forming unit of the virus, and particle formation is independent of pol and env gene products in both mammalian and insect cell expression systems (Gheysen, et al., Cell, 59:103-112 (1989); Karacostas, et al., Proc. Natl. Acad. Sci. USA, 86:8964-8967 (1989); Delchambre, et al., EMBO. J., 8:2653-2660 (1989); Rasmussen, et al., Virology, 178:435-451 (1990); Karacostas, et al., Virology, 193:661-671 (1993); Morikawa, et al., Virology, 183:288-297 (1991); Wagner, et al., Arch. Virol., 127:117-137 (1992)). For HIV, expression of Pr55 by recombinant baculoviruses or vaccinia viruses results in the formation of nonreplicating, noninfectious, virus-like particles, or Gag pseudovirions, that lack genomic length viral RNA. The production of Gag pseudovirions in mammalian and insect cell systems using recombinant virus vectors provides a novel technology for engineering recombinant protein-based particulate vaccines for HIV and other viruses (Gheysen, et al., Cell, 59:103-112 (1989); Karacostas, et al., Proc. Natl. Acad. Sci. USA, 86:8964-8967 (1989); Delchambre, et al., EMBO. J., 8:2653-2660 (1989); Rasmussen, et al., Virology, 178:435-451 (1990); Karacostas, et al., Virology, 193, 661-671 (1993); Morikawa, et al., Virology, 183:288-297 (1991); Wagner, et al., Arch. Virol., 127:117-137 (1992)).
Recently, efforts have focused on developing strategies to package additional viral peptides and polyproteins into HIV Gag pseudovirions using the baculovirus insect cell expression system. It is thought that the incorporation of additional viral peptides and polyproteins may be advantageous in vaccine preparations, since they may contain important antigenic epitopes that may play a role in inducing protection from infection or disease. To date, however, such efforts have not been fully successful.